1. Field of the Invention
The present invention relates to a semiconductor device using a refractory metal as an electrode and interconnection. Particularly, the present invention relates to a structure of a metal interconnection and electrode in a very large scale integrated (VLSI) circuit device.
2. Description of the Prior Art
FIGS. 1A to 1E are sectional views showing major steps of manufacturing of a conventional semiconductor device using a refractory metal as an electrode and interconnection. In the following, a method of manufacturing of a conventional semiconductor device will be described with reference to FIGS. 1A to 1E.
Referring to FIG. 1A, thick oxide films 2a and 2b serving as cell isolation regions are formed selectively in predetermined regions on a major surface of a silicon semiconductor substrate 1 by using a thermal oxidation process or the like. Then, a thin gate oxide film 3 is formed on the semiconductor substrate 1 by a thermal oxidation process and after that, an impurity 4 such as boron, phosphorous or arsenic is implanted through the gate oxide film 3 so as to control a threshold voltage of a MOS (metal-oxide-semiconductor) type transistor.
Referring to FIG. 1B, a polycrystalline silicon film 5 is formed on the oxide films 2a, 2b and 3 by using a CVD process or the like and a refractory metal silicide film 6 of titanium silicide or tantalum silicide is further formed on the polycrystalline silicon film 5 by using a sputtering process, a vacuum evaporation process, a CVD process or the like. Then for the purpose of patterning the polycrystalline silicon film 5 and the refractory metal silicide film 6, a resist film 8 is selectively provided in a predetermined region by a photolithography process.
Referring to FIG. 1C, etching is applied using the resist film 8 as a mask so that the polycrystalline silicon film 5 and the refractory metal silicide film 6 are patterned in a predetermined form. Subsequently, by an ion implantation process, a thermal diffusion process or the like, ion introduction is made using the resist film 8 as a mask so that impurity diffusion layers 9a and 9b are formed. The impurity diffusion layers 9a and 9b serve as source and drain regions of a MOS transistor.
Referring to FIG. 1D, the resist film 8 is removed and after that, for the purpose of electrical separation of an electrode and interconnection, silicon oxide films 10a, 10b and 10c are formed by a CVD process, a sputtering process or the like. Subsequently, contact holes 11a and 11b for providing an electrode and interconnection are formed in predetermined regions by a photolithography and etching process. Then, an impurity such as phosphorous is introduced in self-alignment manner through the contact holes 11a and 11b by a thermal diffusion process, an ion implantation process or the like, for purposes of reduction of contact resistance.
Referring to FIG. 1E, an aluminum-alloy film serving as an electrode and interconnection is formed over the whole exposed surface by a sputtering process, a vacuum evaporation process, a CVD process or the like and after that, the aluminum alloy film is patterned by a photolithography and etching process so that electrode interconnection films 12a and 12b are formed. After the above described steps, an insulating film or a passivation film, not shown, is formed and thus, the manufacturing of a MOS transistor using a refractory metal as an electrode and interconnection is completed.
Recently, integrated circuit devices have tended to be of a high density and a high degree of integration and accordingly, it happens that an RC delay in signal (R: resistance, C: parasitic capacitance) is caused by an interconnection resistance of a device, resulting in lowering of the operation speed of the device. Under the circumstances, in order to overcome the above stated drawback, there is an increasing tendency to use as a material for a gate electrode and interconnection, a refractory metal silicide having a low specific resistance (that is, an alloy of a refractory metal and silicon) in place of polycrystalline silicon widely utilized in the conventional art. Among refractory metal silicides, a silicide of molybdenum Mo and a silicide of tungsten W are excellent in corrosion resistance to hydrofluoric acid, but they are both disadvantageous in that the specific resistance is relatively high.
Therefore, approaches are being made to lower the resistance of an electrode and interconnection by using a silicide of titanium or tantalum having a lower specific resistance. However, the manufacturing processes of a semiconducuctor wafer of LSI (large scale integrated circuit) include various chemical treatment steps and therefore, it is particularly necessary for the electrode and interconnection to have an excellent characteristic in corrosion resistance to chemicals containing hydrofluoric acid. For example, in the step shown in FIG. 1E, a light-etching process is applied using chemicals containing hydrofluoric acid before the aluminum-alloy films 12a and 12b are formed. In this light etching, the oxide films (of phosphosilicate glass or silicon oxide or the like) formed as a result of thermal diffusion of an impurity such as phosphorous and the natural oxide films, formed on the surfaces of the impurity diffusion layers 9a and 9b, are completely removed so that the aluminum alloy films 12a and 12b can be stably brought into contact with a gate electrode interconnection film (a film comprised of polycrystalline silicon 5 and a refractory metal silicide 6) and the impurity diffusion layer 9b, respectively, to lower the contact resistance. However, in this light etching it is necessary not to etch the underlying gate electrode interconnection film. This is based on the below described reasons. The contact resistance consists of a resistance in contact between the refractory metal silicide 6 and the aluminum-alloy film 12a and an increase of resistance associated with an increase of a current density in the refractory metal silicide film 6 in the contact hole. Therefore, in the etching process, as the thickness of the refractory metal silicide film 6 in the contact hole becomes thin, the current density in the refractory metal silicide film 6 increases largely as compared with the case of the film 6 being thick, and as a result, electric current cannot flow easily and the contact resistance is increased. Furthermore, if the low resistance refractory metal silicide film 6 is completely removed and the underlying polysilicon film 5 is exposed, the contact resistance increases excessively.
FIGS. 2A and 2B are views showing a light etching process in a gate electrode interconnection film. The above stated problem will be considered in the following with reference to FIGS. 2A and 2B, the portions corresponding to those in FIGS. 1A to 1E being denoted by the same reference numerals.
Referring to FIG. 2A, an oxide film (a silicon oxide film or phosphosilicate glass) 13 formed on the refractory metal silicide film 6 is removed by a light etching process using chemicals containing hydrofluoric acid. The refractory metal silicide film 6 as the underlying base for the oxide film 13 is formed of titanium silicide or tantalum silicide having a low resistance and it has an inferior characteristic in corrosion resistance to chemicals containing hydrofluoric acid.
Referring to FIG. 2B, when the underlying electrode interconnection film 6 is formed of titanium silicide or tantalum silicide, the selectivity of etching with chemicals containing hydrofluoric acid with respect to the oxide film 13 and the underlying refractory metal silicide film 6 is not satisfactory, and therefore the underlying refractory metal silicide film 6 is etched excessively. As a result, the thickness of the refractory metal silicide film 6 becomes thinner or in a further case the underlying silicon film 5 is exposed, resulting in an increase in the sheet resistance and the contact resistance of the metal silicide film 6.
On the other hand, although molybdenum silicide MoSi.sub.2 and tungsten silicide WSi.sub.2 are excellent in corrosion resistance to chemicals containing hydrofluoric acid, they are disadvantageous in that the specific resistance values of WSi.sub.2 and MoSi.sub.2 are respectively 70 .mu..OMEGA.cm and 100 .mu..OMEGA.cm, which are higher than the specific resistance values 1.5 .mu..OMEGA.cm and 30 .mu..OMEGA.cm of titanium silicide TiSi.sub.2 and tantalum silicide TaSi.sub.2 respectively. The physical and chemical properties of refractory metal silicides are described for example in "Refractory silicides for integrated circuits", by S. P. Murarka, American Vacuum Society #J. Vac. Sci. Technol., Vol. 17, No. 4, Jul./Aug. 1980, pp. 775 to 792.
A method in which a titanium film Ti is in advance covered with a molybdenum film Mo for the purpose of preventing oxidation of the surface of Ti at the time of siliciding the titanium on a silicon substrate is disclosed in "Mo/Ti bilayer metalization for a self-aligned TiSi.sub.2 process", by H. K. Park et al., American Vacuum Society #J. Vac. Sci. Technol. A, Vol. 2, No. 2, Apr.-June 1984, pp. 259 to 263. In this prior art, Mo is not silicided.